The present invention relates generally to seals used in gas turbine engines and, more particularly, to an interstage seal configuration used to reduce secondary flows between rotor wheel-space cavities.
It is well known that turbines extract energy from a hot gas stream as it impinges on the turbine blades mounted on a rotor wheel or disk fixed on a shaft or rotor of an associated rotary apparatus such as a generator. The blades are in the form of airfoils manufactured from materials capable of withstanding extreme temperatures. The mounting and shank portions of the blades are typically made of the same material, but the rotor disk posts between the mounting portions (or dovetails) are made of less capable material. For this reason, it is important to protect the disk posts from the direct impact of the high temperatures of the hot gas stream. Therefore, the blades and adjacent vane elements of the turbine are provided with platforms which axially combine to define a circumferential boundary, thus isolating the radially inner mounting or shank portions from the hot gas stream.
Protection against high temperatures is equally important throughout the rotor cavity. However, it becomes even more pronounced in the interstage region of the high pressure portion of turbine where the boundary of the expanding hot gases comes close to temperature sensitive areas of the rotor cavity, such as the forward and aft cavities bounded by the disk post for the stage one blade wheel, the platform for the stage two stationary nozzle assembly, and by the disc post of the stage two blade wheel.
According to present practice, labyrinth-type seals are often used between the forward and aft cavities. Such seals are well known in the art and include a plurality of circumferential teeth which are contiguous with a circumferential sealing surface made from a high temperature resistant abradable material in, for example, honeycomb form, providing the sealing surfaces with which the labyrinth teeth contact and, due to the deformability of the honeycomb material, the sealing surfaces becomes deformed without injury to the teeth, thereby establishing a minimum clearance required under operating conditions. See, for example, U.S. Pat. No. 5,215,435. Such seals also prevent performance loss due to flow bypassing the stationary airfoils by flowing through the wheel space instead.
Traditional diaphragm and honeycomb carrier designs have a substantially constant inner diameter which requires more radial space for packaging, since the flowpath outboard of the seal is conical in shape. In addition, such designs also involve more intersegment leakage because there is a larger radial gap between the seal teeth on the rotor and the stationary nozzle due to the relatively thick carrier and larger radial height.
Alternatively, some designs have used a cylindrical, sheet metal carrier of uniform diameter, where steps are machined into the honeycomb material.
The problem here is that such machining without damaging the honeycomb material is difficult and, therefore, more expensive and time-consuming methods must be used.
There remains a need therefore, for an interstage seal of simpler construction that also provides improved clearances and sealing over the prior design.